In today's gas turbines it is an aim to burn the fuel in the combustion chamber in a lean mixture of air and fuel. Such kind of gas turbine combustors may be called dry low emission (DLE) combustion systems, whereby the combustion of the lean fuel mixture produces low NOx (nitrogen oxides) rate and compact flames. However, these systems are prone to combustion dynamics as they run in a lean regime due to the use of the lean mixture of air and fuel. Hence, combustion dynamics may arise as a result of flame excitation, aerodynamic induced excitation or insufficient damping.
The combustion dynamics may cause high acoustic noises wherein it is an aim to reduce those noises, in particular the sound that is generated by the dry low emission combustion systems.
Therefore, in conventional gas turbines, acoustic damping of the critical frequency is performed. Thus, damping devices are installed that are placed directly on the combustion chamber or inside the casings of the gas turbines. The damping devices may be formed of Helmholtz resonator dampers or perforated liners.
Helmholtz resonators are known to be very effective at damping a critical frequency experienced by the gas turbine system. Normally, the Helmholtz resonators are designed to target a critical frequency experienced at a single load point of the gas turbine. When the load of the gas turbine is altered, in particular for example between 50% and 75%, the combustion system might be prone to the combustion dynamics.
In conventional gas turbines, a set of a plurality of Helmholtz resonators with different resonating frequencies are installed that are used to damp different frequencies generated by the combustion dynamics. With this approach, a high number of parts and installation space is required. Moreover, the use of a plurality of Helmholtz resonators might not always be desirable due to geometrical constraints of the gas turbine.
FIG. 4 illustrates a prior art combustion system 400. The combustion system 400 comprises a combustion chamber 401 in which the injected fuel is burnt for generating thermal energy. At an axial end of the tubular-formed combustion chamber 401, the combustion chamber 401 comprises a radial extending pilot face 402. Fuel is injected within the combustion chamber 401 in two or more fuel streams, namely the main fuel stream 405 and the pilot fuel stream 403. The main fuel stream 405 is introduced by a swirler 404, wherein the main fuel stream 405 is introduced in a tubular manner, so that the main fuel is mixed with e.g. air sufficiently until it reaches the flame inside the combustion chamber 401. The pilot fuel stream 403 that is injected inside the combustion chamber 401 from the pilot face 402 streams generally in axial direction in order to guide the main fuel stream 405 in a predetermined direction. The pilot fuel stream 403 has a fuel/air mixture which results in a greater flame stability but with a higher NOx-concentration. The combustion system 400 is generally designed to operate at an optimum between the acceptable levels of combustion dynamics, which comprise generally a key frequency under a predetermined limit, and corresponding Nox-emissions.
EP 0 974 788 A1 discloses a device for reducing sound within a streaming machine. A streaming channel connects a Helmholtz resonator volume with a combustion chamber. Within the Helmholtz resonator volume air and water is injected. In the streaming channel between the Helmholtz resonator volume and the combustion fuel is injected. The fuel pipe may comprise an additional Helmholtz resonator volume.
EP 0 577 862 discloses an after-burner for a gas turbine chamber. The air for the combustion in the combustion chamber is guided through a Helmholtz resonator. After passing the Helmholtz resonator, fuel is injected to the combustion air.
EP 1 004 823 A2 discloses a damping device for reducing an amplitude of acoustical waves inside a burner. The combustion air is guided through a Helmholtz resonator. After passing the Helmholtz resonator, pilot fuel is injected to the combustion air.
GB 246 657 A discloses a turbine engine fuel injector with Helmholtz resonator. Inside an annular ring a plurality of fuel injector nozzles are installed, wherein the fuel streams through smaller and larger sized streaming volumes before being injected into a combustion chamber.
EP 0 597 138 B1 discloses a combustion chamber for a gas turbine. Before the combustion air is injected into a pre-chamber of the combustion chamber, the air flows through a Helmholtz resonator. Fuel is separately injected directly to the pre-chamber.
U.S. Pat. No. 7,320,222 B2 discloses a burner for a gas turbine. The volume of a Helmholtz resonator is connected to a fuel pipe. The gas flow streams through the fuel pipe without flowing through the Helmholtz resonator.